I. Field of the Invention
This invention relates to a method for preparing aqueous dispersions of polytetrafluorethylene (PTFE).
II. Description of the Prior Art
The polymerization of tetrafluoroethylene (TFE) in emulsified form is a known technique which results in aqueous dispersions of polytetrafluoroethylene, the latter being useful for numerous industrial applications including the coating of metals and other substrates, in impregnating fibers and fabrics, in the preparation of coatings and lacquers and for the production of powders by coagulation for lubricated extrusion.
Aqueous dispersions of polytetrafluoroethylene are prepared according to processes now in use by polymerizing a gaseous monomer (or mixture of different monomers where copolymers are sought) with an aqueous solution containing a polymerization initiator susceptible of forming free radicals at the polymerization temperature, optionally a buffering agent to maintain the pH of the reaction medium at a given level and an emulsifying agent which is generally an alkaline or ammonium salt of a carboxylic or sulfonic acid in which the carbon chain contains only fluorine, and optionally, chlorine atoms, said polymerizing step being carried out in an autoclave resistant to pressure.
The polytetrafluoroethylene dispersions obtained by these known methods are poorly stable and it is scarcely possible to obtain a concentration of polytetrafluoroethylene greater than 10 to 15%.
A significant improvement can be obtained according to French Pat. No. 1,019,304 of May 19, 1950 by adding as a stabilizing agent to the polymerization medium, from 0.1 to 12% by weight of the aqueous disperson of a saturated hydrocarbon containing more than 12 carbon atoms which hydrocarbon is liquid under the polymerization conditions.
However, this process presents the disadvantage of rendering the cleaning of the polymerization autoclave difficult leading to down-time which is incompatible with industrial production, particularly in the case when a paraffinic hydrocarbon which is a solid at ambient temperature is used. Moreover, if the hydrocarbon employed in the process is a liquid, its total removal from the dispersion requires rather time-consuming decantation procedures which in practice lead to a significant loss of polytetrafluoroethylene. Finally, if the removal of hydrocarbon is not perfectly complete, residual traces thereof can cause contamination of the final product.
Another improvement, proposed by Belgium patent No. 678,637 of March 29, 1966 comprises using as a stabilizing agent, a paraffinic halogenated hydrocarbon having 1 to 3 carbon atoms, in particular, 1, 1, 2 - trifluoro - 1, 2, 2 - trichloroethane. However, the use of this process is limited because of the necessity of having to employ polymerization temperatures between 5 and 30.degree. C. as well as having to use large quantities of the chlorofluorinated hydrocarbon, namely, from 50 to 500 gm/liter of aqueous dispersion. The process thus requires recovery of the stabilizer by decanting the dispersion but in practice, this leads to a loss of stabilizer.